Wire saws have gradually replaced the internal diameter rotating sawing blades for cutting expensive materials that are hard and brittle. Examples of such materials are quartz (for e.g. quartz oscillators or mask blancs), silicon (for e.g. integrated circuit wafers or solar cells), germanium (e.g. as substrate for gallium arsenide solar cells), gallium arsenide (for high frequency circuitry), silicon carbide or sapphire (e.g. for blue led or blue led substrates), rare earth magnetic alloys (e.g. for recording heads) and the like. One of the driving forces for this evolution is the desire to reduce the kerf loss in the process. ‘Kerf loss’ is the material loss due to the abrading away of the (expensive) material. Hence, a reduced kerf loss results in a better use of material and considerable financial savings.
At the end of the previous century, multiwire saws have been developed (see e.g. GB1 397 676) that enable the cutting of thin slices (called wafers) out of single block of material. In such a multiwire saw, a single metallic wire—which makes the term ‘multiwire’ saw confusing—is guided over wire guides that keep the wires parallel to one another during sawing in a wire web. However, it is not the wire itself that saws, but an abrasive added to the process.
The abrasive can be added to the wire in the form of a viscous carrier (usually poly ethylene glycol) in which an abrasive (normally silicon carbide) is kept in suspension. The mixture is called ‘a slurry’. The wire entrails the carrier that on its turn drags the abrasive into the cut. The abrasive abrades the block of material while it sticks and rolls in between the wire and the material. This is known as ‘loose abrasive sawing’ which is a kind of ‘third body abrasion’. Both material and wire get abraded away which makes the continuous replacement of wire necessary. Also the slurry degrades as the abrasive particles lose their sharpness. In addition the slurry gets loaded with the swarf of the material and the debris of the wire and must be renewed regularly which brings environmental issues with it. However, the process is gentle to the material and therefore liked very much in the semiconductor industry where subsurface damage is an important issue.
In an attempt to eliminate these consumables it has been proposed to fix the abrasive to the wire in a ‘fixed abrasive sawing wire’. In this way the relative movement between wire and abrasive is zero and hence the wire will not abrade. The relative movement between wire and material is doubled (relative to the loose abrasive sawing) leading to a more efficient sawing. The fixation of the abrasive has been implemented in different ways:
A first route was taken by the technologists that were familiar with the making of metallic based fixed abrasive tools such sawing blades wherein abrasive particles—usually diamonds—are embedded in a nickel coating by electroplating or electroless deposition out of a nickel bath comprising diamond particles. An example is described in EP 0 982 094 wherein a metallic wire, the embodiment of a stainless steel wire is given, is coated with nickel with diamonds. However, the use of nickel baths has raised health concerns as metallic nickel and in particular nickel salts are known to be carcinogenic (reference is made to the 30th amendment of EC Directive 67/548/EEC).
A second route—of which WO 99/46077 is an example—was taken by metallurgists familiar with the brazing of abrasive tooling who applied their know-how on abrasive containing brazes for coating a wire with it. However, brazing results in a considerable thermal loading of the wire which results in a loss of tensile strength of the wire (unless very particular metallic wires are used as a substrate).
A third route was taken by chemically skilled persons that sought to apply their knowledge on organic binders for abrasives for making a fixed abrasive sawing wire. There are numerous examples of which U.S. Pat. No. 6,070,570, EP 1 025 942 and WO 2005/011914 are three. Although this results in a cost effective and efficient way of fixing the abrasive on the—usually metallic—wire the fixation is not that strong and the resulting cutting speeds are lower than those obtained in the routes mentioned above. A surface treatment—such as coating the wire with a brass or copper coating or coating with a primer—is considered advantageous in U.S. Pat. No. 6,070,570.
An alternative surface treatment is suggested in JP 10 328932 wherein the surface of a piano wire is roughened by sandblasting prior to coating with an abrasive containing resin. The abrasive particles—surrounded by resin—may or may not fit into a recess of the roughened surface. No direct contact between abrasive and wire is established.
A notable exception to the use of a metallic wire substrate is WO 2003/041899 wherein manmade organic fibres (Dynema®, Nomex®, Kevlar®) are used as strength member.
Alternative development routes that sought to incorporate the abrasive particles into the skin of the metallic wire are EP 0 243 825 and EP 0 081 697:
EP 0 243 825 describes a method to produce a fixed abrasive sawing wire starting from a steel wire rod and a tube surrounding the rod with a gap in between. The gap is filled with a mixture of metal powder and abrasive particles. The ends are sealed and the rod is heat treated and cold drawn in repeated steps to obtain a fixed abrasive sawing wire after the outer tube has been removed by etching it away. Drawbacks are that the method does not allow to produce fixed abrasive sawing wires of an appreciable length (above 100 meters), the tensile strength of the resulting wire is relatively low (say below 1800 N/mm2) and the resulting wires are too thick (1 mm).
EP 0 081 697 describes a method and an apparatus to incrust a wire with diamond particles. One departs from a wire that is coated with a copper or nickel layer prior to incrustation of diamond particles between hardened wheels that roll the wire around its axis through a repetitive axial movement of one or both of the wheels. Thereafter the diamonds are fixed in position by means of an electrolytically applied overcoat. Apart from the health problems with nickel and nickel salts (op cit), the application of a nickel overcoat necessitates the introduction of a plating section into a continuous process line. It adds considerable length to the overall length of the line and the ancillaries (pumping, tubing, vessels) and safety precautions make it an expensive operation.